X-ray sources

ABSTRACT

An anode for an X-ray source is formed in two parts, a main part ( 18 ) and a collimating part ( 22 ). The main part ( 18 ) has the target region ( 20 ) formed on it. The two parts between them define an electron aperture ( 36 ) through which electrons pass reach the target region ( 20 ), and an X-ray aperture through which the X-rays produced at the target leave the anode. The anode produces at least the first stage of collimation of the X-ray beam produced.

The present invention relates to X-ray sources and in particular to thedesign of anodes for X-ray sources.

Multifocus X-ray sources generally comprise a single anode, typically ina linear or arcuate geometry, that may be irradiated at discrete pointsalong its length by high energy electron beams from a multi-elementelectron source. Such multifocus X-ray sources can be used intomographic imaging systems or projection X-ray imaging systems where itis necessary to move the X-ray beam.

The present invention provides an anode for an X-ray tube comprising atarget arranged to produce X-rays when electrons are incident upon it,the anode defining an X-ray aperture through which the X-rays from thetarget are arranged to pass thereby to be at least partially collimatedby the anode.

The anode may be formed in two parts, and the X- ray aperture canconveniently be defined between the two parts. This enables simplemanufacture of the anode. The two parts are preferably arranged to beheld at a common electrical potential.

Preferably a plurality of target regions are defined whereby X-rays canbe produced independently from each of the target regions by causingelectrons to be incident upon it. This makes the anode suitable for use,for example, in X-ray tomography scanning. In this case the X- rayaperture may be one of a plurality of X-ray apertures, each arranged sothat X-rays from a respective one of the target regions can pass throughit.

Preferably the anode further defines an electron aperture through whichelectrons can pass to reach the target. Indeed the present inventionfurther provides an anode for an X-ray tube comprising a target arrangedto produce X-rays when electrons are incident upon it, the anodedefining an electron aperture through which electrons can pass to reachthe target.

Preferably the parts of the anode defining the electron aperture arearranged to be at substantially equal electrical potential. This canresult in zero electric field within the electron aperture so thatelectrons are not deflected by transverse forces as they pass throughthe electron aperture. Preferably the anode is shaped such that there issubstantially zero electric field component perpendicular to thedirection of travel of the electrons as they approach the anode. In someembodiments the anode has a surface which faces in the direction ofincoming electrons and in which the electron aperture is formed, andsaid surface is arranged to be perpendicular to the said direction.

Preferably the electron aperture has sides which are arranged to besubstantially parallel to the direction of travel of electronsapproaching the anode. Preferably the electron aperture defines anelectron beam direction in which an electron beam can travel to reachthe target, and the target has a target surface arranged to be impactedby electrons in the beam, and the electron beam direction is at an angleof 10° or less, more preferably 5° or less, to the target surface.

Preferably the anode claim further comprises cooling means arranged tocool the anode. For example the cooling means may comprise a coolantconduit arranged to carry coolant through the anode. Preferably theanode comprises two parts and the coolant conduit is provided in achannel defined between the two parts.

The present invention further provides an X-ray tube including an anodeaccording to the invention.

Preferred embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings inwhich:

FIG. 1 is a schematic representation of an X-ray tube according to afirst embodiment of the invention;

FIG. 2 is a partial perspective view of an anode according to a secondembodiment of the invention;

FIG. 3 is a partial perspective view of a part of an anode according toa third embodiment of the invention;

FIG. 4 is a partial perspective view of the anode of FIG. 4; and

FIG. 5 is a partial perspective view of an anode according to a fourthembodiment of the invention.

Referring to FIG. 1, an X-ray tube according to the invention comprisesa multi-element electron source 10 comprising a number of elements 12each arranged to produce a respective beam of electrons, and a linearanode 14, both enclosed in a tube envelope 16. The electron sourceelements 12 are held at a high voltage negative electrical potentialwith respect to the anode.

Referring to FIG. 2, the anode 14 is formed in two parts: a main part 18which has a target region 20 formed on it, and a collimating part 22,both of which are held at the same positive potential, beingelectrically connected together. The main part 18 comprises an elongateblock having an inner side 24 which is generally concave and made up ofthe target region 20, an X-ray collimating surface 28, and an electronaperture surface 30. The collimating part 22 extends parallel to themain part 18. The collimating part 22 of the anode is shaped so that itsinner side 31 fits against the inner side 24 of the main part 18, andhas a series of parallel channels 50 formed in it such that, when thetwo parts 18, 22 of the anode are placed in contact with each other,they define respective electron apertures 36 and X-ray apertures 38.Each electron aperture 36 extends from the surface 42 of the anode 14facing the electron source to the target 20, and each X-ray apertureextends from the target 20 to the surface 43 of the anode 14 facing inthe direction in which the X-ray beams are to be directed. A region 20 aof the target surface 20 is exposed to electrons entering the anode 14through each of the electron apertures 36, and those regions 20 a aretreated to form a number of discrete targets. In this embodiment, theprovision of a number of separate apertures through the anode 14, eachof which can be aligned with a respective electron source element,allows good control of the X-ray beam produced from each of the targetregions 20 a. This is because the anode can provide collimation of theX-ray beam in two perpendicular directions. The target region 20 isaligned with the electron aperture 36 so that electrons passing alongthe electron aperture 36 will impact the target region 20. The two X-raycollimating surfaces 28, 32 are angled slightly to each other so thatthey define between them an X-ray aperture 38 which widens slightly inthe direction of travel of the X-rays away from the target region 20.The target region 20, which lies between the electron aperture surface30 and the X-ray collimating surface 28 on the main anode part 18 istherefore opposite the region 40 of the collimating part 22 where itselectron aperture surface 34 and X-ray collimating surface 32 meet.

Adjacent the outer end 36a of the electron aperture 36, the surface 42of the anode 14 which faces the incoming electrons and is made up on oneside of the electron aperture 36 by the main part 18 and on the otherside by the collimating part 22, is substantially flat and perpendicularto the electron aperture surfaces 30, 34 and the direction of travel ofthe incoming electrons. This means that the electrical field in the pathof the electrons between the source elements 12 and the target 20 isparallel to the direction of travel of the electrons between the sourceelements 12 and the surface 42 of the anode facing the source elements12. Then within the electron aperture 36 between the two parts 18, 22 ofthe anode 14 there is substantially no electric field, the electricpotential in that space being substantially constant and equal to theanode potential.

In use, each of the source elements 12 is activated in turn to project abeam 44 of electrons at a respective area of the target region 20. Theuse of successive source elements 12 and successive areas of the targetregion enables the position of the X-ray source to be scanned along theanode 14 in the longitudinal direction perpendicular to the direction ofthe incoming electron beams and the X-ray beams. As the electrons movein the region between the source 12 and the anode 14 they areaccelerated in a straight line by the electric field which issubstantially straight and parallel to the required direction of travelof the electrons. Then, when the electrons enter the electron aperture36 they enter the region of zero electric field which includes the wholeof the path of the electrons inside the anode 14 up to their point ifimpact with the target 20. Therefore throughout the length of their paththere is substantially no time at which they are subject to an electricfield with a component perpendicular to their direction of travel. Theonly exception to this is any fields which are provided to focus theelectron beam. The advantage of this is that the path of the electronsas they approach the target 20 is substantially straight, and isunaffected by, for example, the potentials of the anode 14 and source12, and the angle of the target 20 to the electron trajectory.

When the electron beam 44 hits the target 20 some of the electronsproduce fluorescent radiation at X-ray energies. This X-ray radiation isradiated from the target 20 over a broad range of angles. However theanode 14, being made of a metallic material, provides a high attenuationof X-rays, so that only those leaving the target in the direction of thecollimating aperture 38 avoid being absorbed within the anode 14. Theanode therefore produces a collimated beam of X-rays, the shape of whichis defined by the shape of the collimating aperture 38. Furthercollimation of the X-ray beam may also be provided, in conventionalmanner, externally of the anode 14.

Some of the electrons in the beam 44 are backscattered from the target20. Backscattered electrons normally travel to the tube envelope wherethey can create localised heating of the tube envelope or build upsurface charge that can lead to tube discharge. Both of these effectscan lead to reduction in lifetime of the tube. In this embodiment,electrons backscattered from the target 20 are likely to interact withthe collimating part 22 of the anode 14, or possibly the main part 18.In this case, the energetic electrons are absorbed back into the anode14 so avoiding excess heating, or surface charging, of the tube envelope16. These backscattered electrons typically have a lower energy than theincident (full energy) electrons and are therefore more likely to resultin lower energy bremsstrahlung radiation than fluorescence radiation.There is a high chance that this extra off-focal radiation will beabsorbed within the anode 14 and therefore there is little impact ofoff-focal radiation from this anode design.

In this particular embodiment shown in FIG. 2, the target 20 is at a lowangle of preferably less than 10°, and in this case about 5°, to thedirection of the incoming electron beam 44, so that the electrons hitthe target 20 at a glancing angle. The X-ray aperture 38 is thereforealso at a low angle, in this case about 10° to the electron aperture 36.With conventional anodes, it is particularly in this type of targetgeometry that the incoming electrons tend to be deflected by theelectric field from the target before hitting it, due to the highcomponent of the electric field in the direction transverse to thedirection of travel of the electrons. This makes glancing angleincidence of the electrons on the anode very difficult to achieve.However, in this embodiment the regions inside the electron aperture 36and the X-ray aperture 38 are at substantially constant potential andtherefore have substantially zero electric field. Therefore theelectrons travel in a straight line until they impact on the target 20.This simplifies the design of the anode, and makes the glancing angleimpact of the electrons on the anode 20 a practical design option. Oneof the advantages of the glancing angle geometry is that a relativelylarge area of the target 20, much wider than the incident electron beam,is used. This spreads the heat load in the target 20 which can improvethe efficiency and lifetime of the target.

Referring to FIGS. 3 and 4, the anode of a second embodiment of theinvention is similar to the first embodiment, and corresponding partsare indicated by the same reference numeral increased by 200. In thissecond embodiment, the main part 218 of the anode is shaped in a similarmanner to that of the first embodiment, having an inner side 224 made upof a target surface 220, and an X-ray collimating surface 228 and anelectron aperture surface 230, in this case angled at about 11° to thecollimating surface 228. The collimating part 222 of the anode again hasa series of parallel channels 250 formed in it, each including anelectron aperture part 250 a, and an X-ray collimating part 250 b suchthat, when the two parts 218, 222 of the anode are placed in contactwith each other, they define respective electron apertures 236 and X-rayapertures 238. The two X-ray collimating surfaces 228, 232 are angled atabout 90° to the electron aperture surfaces 230, 234 but are angledslightly to each other so that they define between them the X-rayaperture 238 which is at about 90° to the electron aperture 236.

As with the embodiment of FIG. 2, the embodiment of FIGS. 3 and 4 showsthat the collimating apertures 238 broaden out in the horizontaldirection, but are of substantially constant height. This produces afan-shaped beam of X-rays suitable for use in tomographic imaging.However it will be appreciated that the beams could be madesubstantially parallel, or spreading out in both horizontal and verticaldirections, depending on the needs of the particular application.

Referring to FIG. 5, in a third embodiment of the invention the anodeincludes a main part 318 and a collimating part 322 similar in overallshape to those of the first embodiment. Other parts corresponding tothose in FIG. 2 are indicated by the same reference numeral increased by300. In this embodiment the main part 318 is split into two sections 318a, 318 b, one 318 a which includes the electron aperture surface 330,and the other of which includes the target region 320 and the X-raycollimating surface 328. One of the sections 318 a has a channel 319formed along it parallel to the target region 320, i.e. perpendicular tothe direction of the incident electron beam and the direction of theX-ray beam. This channel 319 is closed by the other of the sections 318b and has a coolant conduit in the form of a ductile annealed copperpipe 321 inside it which is shaped so as to be in close thermal contactwith the two sections 318 a, 318 b of the anode main part 318. The pipe321 forms part of a coolant circuit such that it can have a coolantfluid, such as a transformer oil or fluorocarbon, circulated through itto cool the anode 314. It will be appreciated that similar cooling couldbe provided in the collimating part 322 of the anode if required.

1. An anode for an X-ray tube comprising a target arranged to produceX-rays when electrons are incident upon it, the anode defining an X-rayaperture through which the X-rays from the target pass through and areat least partially collimated by the anode and wherein the X-rayaperture is one of a plurality of X-ray apertures, each arranged so thatX-rays from a respective one of the target regions can pass through. 2.An anode according to claim 1 wherein the anode is formed in two parts,which define the X-ray aperture.
 3. An anode according to claim 2wherein the two parts are held at a substantially equal electricalpotential.
 4. An anode according to claim 1 wherein a plurality oftarget regions are defined and wherein X-rays are produced independentlyfrom each of the target regions by causing electrons to be incident uponit.
 5. An anode according to claim 1 further defining an electronaperture through which electrons can pass to reach the target.
 6. Ananode for an X-ray tube comprising a target arranged to produce X-rayswhen electrons are incident upon it, the anode defining an electronaperture through which electrons can pass to reach the target.
 7. Ananode according to claim wherein the parts of the anode defining theelectron aperture are at substantially equal electrical potential.
 8. Ananode according to claim 5 wherein the anode is shaped such that thereis substantially zero electric field component perpendicular to thedirection of travel of the electrons as they approach the anode. 9.(canceled)
 10. An anode according to claim 5 wherein the electronaperture comprises sides which are substantially parallel to thedirection of travel of electrons approaching the anode.
 11. An anodeaccording to claim 7 wherein the electron aperture defines an electronbeam direction in which an electron beam can travel to reach the target,and wherein the target has a target surface impacted by electrons in thebeam, and wherein the electron beam direction is at an angle of 10° orless to the target surface.
 12. An anode according to claim 11 whereinthe electron beam direction is at an angle of 5° or less to the targetsurface.
 13. An anode according to claim 1 wherein the anode furthercomprises cooling means for cooling the anode.
 14. An anode according toclaim 13 wherein the cooling means comprises a coolant conduit forcarrying coolant through the anode.
 15. An anode according to claim 14wherein the anode comprises two parts and the coolant conduit isprovided in a channel defined between the two parts.
 16. (canceled) 17.(canceled)
 18. (canceled)
 19. An X-ray tube comprising: an anode furthercomprising a target arranged to produce X-rays when electrons areincident upon it, the anode defining an X-ray aperture through which theX-rays from the target pass through and are at least partiallycollimated by the anode and wherein the X-ray aperture is one of aplurality of X-ray apertures, each arranged so that X-rays from arespective one of the target regions can pass through; and an electronsource.